Among the tracking error detecting systems used for CD are a three beam system and a single beam system including push-pull and differential phase techniques.
The three beam system uses a main beam and two sub-beams and measures a change in the intensity of reflected light from a pitted surface of the disk by means of a photodetector. Despite of reliable operation, the three beam system requires a diffraction grating or the like due to the use of sub-beams, adding to the complexity of the optical system and is less effective in the utilization of laser output. Since the main beam and the sub-beams must be separated, only a limited area of the photodetector is available for RF signals so that the photodetector may not fully receive scattering light. The system is thus weak against such defects as fingerprints and fogs on the disk.
In contrast, the single beam system including push-pull and differential phase techniques imposes no limit on the area of a photodetector which can receive more scattering light as compared with the three beam system, reducing a drop of RF signal.
Among the single beam system, the push-pull technique acquires a tracking error signal by utilizing the fact that the intensity distribution of a light beam which has been diffracted and reflected by a pit and re-entered the objective changes with a relative positional change between the pit and a beam spot. Using a photodetector divided into two segments, the difference between the segments is determined. When the beam is positioned coincident with the information format, the intensity distribution is equal between the left and the right. When this positional relationship is offset, the intensity distribution becomes asymmetric between the left and the right. Since this asymmetry is reversed depending on the positional relationship between the beam and the pit, one polarity corresponding to the beam offset to one side of the pit is available as a tracking error signal.
Nevertheless, the push-pull technique has several drawbacks. No tracking error signals are available at a pit depth equal to .lambda./4 wherein .lambda. is a wavelength because the intensity distribution becomes an metric pattern at that pit depth. When a pickup of the slide type is used, a DC offset is introduced into the tracking error signal. Thus in the event of skew adjustment, automatic gain adjustment should be carried out by causing a sine wave of minute amplitude to overlap the tracking error signal.
The differential phase technique of the single beam system was proposed in order to eliminate the drawbacks of the three beam system and the push-pull technique. (See National Technical Report, Vol. 32, No. 4, Aug. 1986.)
The differential phase technique is to detect a tracking error from a phase difference which is introduced in a photodetector signal by the positional relationship between a pit and a beam spot. A tracking error signal is acquired by determining the intensity distribution of reflected light from the pit through Fourier conversion, carrying out ray tracing using the reflected light as an incident ray, determining the intensity distribution on a photodetector which is divided into four segments, converting diagonally added signals from the photodetector into digital waveforms by means of comparators, respectively, and passing pulses corresponding to the phase difference between the digital waveforms through an integration circuit for converting them into an analog waveform.
There was proposed an optical recording disk which is once-writable or recordable in accordance with the CD standard. See Nikkei Electronics, Jan. 23, 1989, No. 465, p. 107; the Functional Dye Department of the Kinki Chemical Society, Mar. 3, 1989, Osaka Science & Technology Center; and SPIE, Vol. 1078, Optical Data Storage Topical Meeting, 80, 1989.
This disk has a dye layer, a reflective gold layer, and a protective layer disposed on a transparent resin substrate in this order. That is, the reflective layer is in close contact with the dye layer. As opposed to the prior art disk of the structure wherein an air space is provided on a dye layer in order to allow pits to be formed in the dye layer, the newly proposed disk is of the close contact type wherein the reflective layer is close to the dye layer, meeting the total disk thickness of 1.2 mm required by the CD standard. It is to be noted that the substrate is formed with a spiral groove for tracking at a pitch of about 1.6 .mu.m, with that portion of the dye layer in the groove serving as a recording track.
If the differential phase technique is applied to such optical recording disks for tracking error detection, there arise serious problems including a low tracking control loop gain, a low output level available to the servo system which becomes unstable, an increased access time associated with chapter search, and inaccurate chapter search resulting in search errors.